1. Field of the Invention
This invention pertains to an implant for passing blood flow directly between a chamber of the heart and a coronary vessel. More particularly, this invention pertains to such an implant formed of pyrolytic carbon and a method of making such an implant.
2. Description of the Prior Art
Commonly assigned and co-pending U.S. patent application Ser. No. 08/882,397 filed Jun. 25, 1997, now U.S. Pat. No. 5,944,019, entitled "Method and Apparatus for Performing Coronary Bypass Surgery", and filed in the name of inventors Mark B. Knudson and William L. Giese (also filed as PCT application Ser. No. PCT/US97/13980 published Feb. 25, 1998), teaches an implant for defining a blood flow conduit directly from a chamber of the heart to a lumen of a coronary vessel. An embodiment disclosed in the aforementioned application teaches an L-shaped implant in the form of a rigid conduit having one leg sized to be received within a lumen of a coronary artery and a second leg sized to pass through the myocardium and extend into the left ventricle of the heart. As disclosed in the above-referenced application, the conduit is rigid and remains open for blood flow to pass through the conduit during both systole and diastole. The conduit penetrates into the left ventricle in order to prevent tissue growth and occlusions over an opening of the conduit.
Commonly assigned and co-pending U.S. patent application Ser. No. 08/944,313 filed Oct. 6, 1997, now U.S. Pat. No. 5,984,956, entitled "Transmyocardial Implant", and filed in the name of inventors Katherine S. Tweden, Guy P. Vanney and Thomas L. Odland, teaches an implant such as that shown in the aforementioned '397 application with an enhanced fixation structure. The enhanced fixation structure includes a fabric surrounding at least a portion of the conduit to facilitate tissue growth on the exterior of the implant.
Implants such as those shown in the aforementioned applications include a portion to be placed within a coronary vessel and a portion to be placed within the myocardium. Such implants may be made of titanium. Titanium has a long history for use as an implant material in contact with blood. For example, titanium is used as a material in heart valves because it is thrombo-resistant.
Another desirable material for use in blood-contacting implants is pyrolytic carbon. Long used in heart valves, pyrolytic carbon is biocompatible and thrombo-resistant.
In the heart valve industry, pyrolytic carbon is used to coat the external, blood contacting surfaces component parts. For example, a part, such as a valve leaflet, is first formed of a substrate material with dimensions smaller than the desired part but conforming in geometry to the desired part. A common substrate material is graphite because pyrolytic carbon adheres to graphite and graphite is resistant to thermal expansion and can tolerate the extreme temperatures experienced when applying pyrolytic carbon.
The graphite substrate is placed in a fluidized bed reactor with a reaction zone having temperatures of about 1,300.degree. C. Propane gas is used as a carbon source. The carbon is deposited on the external surface of the part as pyrolytic carbon. The coating continues for a time selected to achieve a desired thickness of the pyrolytic carbon so that the final part has a size approximate to the desired size of the part. The pyrolytic carbon layer assumes the geometry of the external surface of the graphite substrate. If necessary the pyrolytic carbon can be machined. For certain applications, it is desired for the pyrolytic carbon to be alloyed with silicon to enhance wear resistance. In such cases, methyltrichlorosilane is added to the reactor as a silicon source during the fluidized bed reaction.
The afore-said applications disclose the desirability of a pyrolytic or pyrolytic-coated titanium implant. However, for use as such an implant, the interior surface of such an implant should be pyrolytic carbon as opposed to the external surfaces of valve components. The afore-mentioned process for coating external surfaces with pyrolytic carbon do not suggest how to form an implant with an internal surface of pyrolytic carbon. Further, a typical such implant has a curved interior surface further frustrating attempts to fabricate or coat the implant with pyrolytic carbon. Also, the long, narrow (i.e., the diameter is less than the length) blood flow path frustrates attempts to so fabricate or coat.